1. Field of the Invention
The present invention relates to a method of increasing an index of refraction of silica glass induced by radiation, which is applicable to a portion having high index of refraction required of an optical component such as an optical waveguide or a fiber optic diffraction grating.
2. Description of the Related Art
Presently, it is known that the index of refraction of silica glass can be increased by induction by radiation of light. Here, the term "light" is used to represent wide concept including not only visible light but also light of every wavelength.
In HOSHAKOU (Synchrotron Radiation) Vol. 7, No. 2 (1994) pp. 112-114, it is described that the index of refraction of silica glass can be increased by exciting valence electrons (electrons of the outermost shell) of silica glass by radiation. It is considered that the change in the index of refraction derives from distortion of atomic arrangement and lattice defect generated by radiation in silica glass.
FIG. 6 is a graph showing relation between wavelength of light and change in the index of refraction disclosed in Japanese Patent Laying-Open No. 6-258674. In this graph, the abscissa represents the light wavelength .lambda. (nm) directed to the silica glass, and the ordinate represents an amount of change .DELTA.n.times.10.sup.3 in the index of refraction of the irradiated silica glass. As can be seen from FIG. 6, it is found in Japanese Patent Laying-Open No. 6-258674 that remarkable change in the index of refraction is observed in silica glass when it is irradiated with light having the wavelength of about 150 nm.
Now, in order to obtain desired change in the index of refraction necessary for forming an optical part, light radiation for about several ten minutes are necessary even when light having the wavelength of about 150 nm which causes such a remarkable change in the index of refraction shown in FIG. 6 is used. Such a long period of time for irradiation presents a problem of inefficiency, in view of industrial processing.
As described in column 2, paragraph number [0013] of Japanese Patent Laying-Open No. 6-258674, the change in the index of refraction of silica glass is not very large when it is irradiated with light having the wavelength longer than 160 nm, but the thickness range in which the index of refraction changes becomes as large as several mm, which is considerably large. On the other hand, if the silica glass is irradiated with light having a wavelength shorter than 160 nm, the change in the index of refraction is large, but the range in thickness in which the index of refraction changes becomes as small as several nm at the surface. Therefore, it is difficult to precisely adjust the distribution of the change in index of refraction in the order of .mu.m in the thickness direction of silica glass.